1. Field of the Invention
The present invention relates generally to a wireless communication system, and specifically to a non-linear, constant envelope modulator and transmit architecture for a wireless communication system.
2. Description of the Related Art
Mobile wireless phones have gained widespread use throughout the world. These phones typically utilize cellular network systems, such as PHS, having base stations serving a predetermined area or cell, where each base station has a certain number of channels for communicating with handsets within that cell. The transmitted signals undergo some type of modulation to transmit a signal on an available channel within the predetermined cell, where the modulated signals are amplified to the proper power level for transmission. The power levels of the signals transmitted by the handsets are precisely controlled by power amplifiers in order to prevent signal interference from occurring at the base station with signals received from other handsets, while also ensuring a quality signal is transmitted. Digital modulators, such as a .pi./4 differential quadrature phase shift-keying (DQPSK) modulator, are typically used for PHS handsets and other digital wireless phones. A .pi./4 DQPSK modulation signal contains both phase and amplitude information, which requires the transmit chain of the handset to maintain a certain level of linearity in order to meet system requirements of adjacent channel power and transmit accuracy. The linearity requirements on the power amplifier require it to be operated in a moderately linear power mode, such as a Class AB mode. Moderately linear power modes are less efficient than highly non-linear power modes and consume a larger amount of current to operate. Thus, the large amount of current consumed by a handset using a linear power amplifier reduces the talk time of the handset or requires the use of a large battery.
Referring now to FIG. 1, a conventional modulator and transmit architecture for a wireless handset is illustrated. Information to be transmitted by the handset is fed into a differential encoder 10, where an in-phase component (I) and a quadrature component (Q) of the modulation signal to be transmitted is created. The I and Q components are then passed through digital filters 12 which give the modulation a particular shape. The resultant I and Q filtered signals are then modulated at a radio frequency for transmission at mixers 16 and combined as a .pi./4 DQPSK modulation signal. The .pi./4 DQPSK modulation signal is then amplified to bring the signal to a desired power level for transmission.
This type of conventional modulator and transmit architecture shown in FIG. 1 necessarily operates using a moderately linear power amplifier to maintain the requisite level of linearity, which undesirably operates the handset in a non-efficient mode. The talktime of the handset could be increased if a more efficient, non-linear power amplifier were utilized in the transmit architecture. However, the in-phase and quadrature components of the modulation signal and, thus, the phase modulation signal will have both phase and amplitude modulation (AM) components. Thus, both the phase and amplitude modulation components of the signals are fed through the power amplifier. When an AM envelope signal is amplified with a nonlinear power amplifier, the AM envelope on the output of the amplifier is distorted due to the spectral spreading of the AM envelope signals which will occur in the power amplifiers. Therefore, non-linear power amplifiers can not be used with current modulator and transmit architectures without causing spectral spreading and violating channel power specifications.
There is a need for a constant envelope modulator and transmit architecture which allows an efficient, non-linear power amplifier to be used to amplify the signal to a desired power level. Moreover, there is a need for a constant envelope modulator and transmit architecture which does not amplify the amplitude information on the modulation signal when amplifying the modulation signal to a desired power level in order to prevent spectral spreading resulting from amplification of the amplitude information on the modulation signal.